The invention relates to a high-current, low-voltage multipole circuit breaker with high electrodynamic strength. In the past, high-current circuit breakers (for indication purposes between 630 A and 6300 A) acting as base switchgear apparatuses for the incomers and feeders in large power installations, were formed by composite elements assembled on a metal frame, whence them being given the name of "open" power circuit breakers. But progressively the equipment of this range inherited of part of the technology of lower power circuit breakers, called "molded case" circuit breakers because they are characterized by an insulating protective enclosure, generally molded in reinforced polyester, housing the poles with their extinguishing chambers, and an operating mechanism and trip devices. The protective enclosure, by contributing to ensuring confinement of breaking and limitation of its external effects, integral partitioning between poles and a better insulation between the power circuit and the auxiliaries, in return enabled the overall dimensions of these apparatuses to be reduced.
The document EP-A-0,322,321 describes a circuit breaker of this type, whose case is formed by assembly of an intermediate case, of the cover forming the circuit breaker front panel, and of a rear panel. The front face of the intermediate case divides the case into a front compartment bounded by this face and by the cover, and a rear compartment designed for housing the poles and electrically insulated from the front compartment. The front compartment houses an operating mechanism acting on a transverse switching shaft common to all the poles, called the pole shaft. This shaft is supported by bearings fitted on the front face of the intermediate case. The rear compartment is for its part subdivided by insulating separating partitions into individual compartments for housing the poles. The front wall of the intermediate case comprises in addition, for each pole, an aperture for access to the corresponding individual compartment. Each pole comprises a pair of separable contacts with a stationary contact and a movable contact, and an arc extinguishing chamber. Each movable contact is mechanically linked to the transverse shaft by means of a connecting rod passing through the front wall of the intermediate case via the corresponding access aperture.
Each rod connecting one of the movable contacts to the transverse shaft is arranged in such a way that in the closed position of the contacts, and in a plane of straight cross-section perpendicular to the pivoting axis of the pole shaft, the distance between a straight line passing through the rotation axes of the connecting rod and the pivoting axis of the shaft is small. In other words, the leverage of the resultant of the forces exerted by the contacts on the pole shaft is small which guarantees that the connecting rod, when it transmits large electrodynamic forces, only generates a low torque at the level of the shaft. At static equilibrium in the closed position of the contacts, the operating mechanism exerts on the shaft a torque opposing the electrodynamic forces transmitted by the connecting rods. This torque only generates low forces at the level of the operating mechanism. Moreover, the resultant of the reaction forces at the level of the guide bearings of the shaft is great and opposes the forces transmitted by the connecting rod and by the operating mechanism.
This architecture is characteristic of circuit breakers with high electrodynamic strength. These circuit breakers must in fact by definition, in order to achieve time selectivity in the electrical installation, be able to withstand the flow of established fault currents which generate large electrodynamic forces tending to separate the contacts. The relative arrangement of the pole shaft, of the connecting rods with the movable contacts and of the connecting rod to the operating mechanism must be such that these forces do not give rise to separation of the contacts or to opening of the operating mechanism. In this case, the arrangement chosen enables these forces to be transmitted to the case by means of the shaft bearings so that the operating mechanism is not subjected to too great forces or torques.
However, guiding of the pole shaft and transmission of the forces to the circuit breaker case are not completely satisfactory. The transverse shaft must in fact be dimensioned, disposed and supported in such a way that deformation thereof is limited and does not hinder its operation. Furthermore, the pole shaft bearings need to be well secured in the case as the large forces transmitted to them tend to tear them away from the front face of the intermediate case to which they are fixed. Making the assembly rigid imposes the use of costly and bulky fixing parts and bearings and of complementary arrangements on the case. Assembly of the circuit breaker requires a large number of parts resulting in a high cost price and fastidious fitting. This architecture moreover limits miniaturization of the circuit breaker.
Moreover, the numerous openings for passage of the connecting rods between the pole shaft and each of the poles are detrimental to the tightness of the extinguishing chambers. However the electrical arc and the endothermal vaporizations generated by this arc at the level of certain elements of the extinguishing chamber partitions give rise to an overpressure and of a gas flow which has to be channeled towards the outlet orifices provided with suitable filters. In order not to hamper inlet of the arc to the extinguishing chamber, it is judicious to place these outlet orifices at the bottom of the extinguishing chambers. The presence of the openings for passage of the connecting rods, situated just above the contacts at the inlet of the chambers, therefore considerably hampers the flow of the gases to the outlet orifices. It allows an uncontrolled gas flow through the front compartment and the openings of the front face, directly to the outside, without any protective filter.